1. Field of the Invention
The present invention relates to a character image generating circuit of a bit map system.
2. Description of the Prior Art
Generally, a character image generating circuit of a bit map system has a full page bit map memory which temporarily carries character image data of one full page in a dot pattern comprising "1s" and "0s". An output device, such as a printer, connected to the bit map memory visualizes the image data by providing dots at places where "1s" are located. Therefore, the "character image data" not only carries the information of characters, but also it carries information of specific style of the characters, e.g., whether they are italic, gothic, elite, etc.
A character image generating circuit employing a bit map memory is schematically illustrated in FIG. 1. As shown, a host computer 1, such as a word processor or a computer, supplies a "character code data" to a character generator 3 through a host interface 2.
In contrast to "character image data", the term "character code data" only carries the information of characters. Therefore, the amount of data necessary to represent one character by the character image data is greater than that represented by the character code data. Here, the term "character" includes not only an alphabet or a number, but also a symbol or a sign used in a sentence or a mathematical formula and various graphic information.
The character code data is supplied and stored in a text buffer 4. When character code data of one full page is stored in the text buffer 4 in the form of series of character code data, the series of character code data is applied to a font memory 5 in which each character code data is converted into a character image data of a dot pattern having a specific character figure. The character image data from font memory 5 is then applied to a bit map memory 6 until one full page is stored.
The character image data stored in the bit map memory is applied through a parallel-serial converter 7 to a printer, for example, a laser printer including a laser optical system 8 and an electrophotographic system 9, which is operable to make a print of the character image data stored in the bit map memory.
The character image generating circuit employing the bit map memory is advantageous in that fine control is possible to provide all details of the characters and also to provide the printing position and the printing direction.
However, according to the prior art character image generating circuit employing the bit map memory, the bit map memory must have a very large storage capacity corresponding to one full page. For example, in order to print an A3 size paper at a resolution of 480 dpi (dots per inch), it is necessary to use a RAM (random access memory) of about 5.7 megabytes for the bit map memory, which RAM is very high in cost. As the resolution and/or paper size increases, (for example, the paper size increases from A4 size to A3 size), the cost of the bit map memory increases remarkably.